Noise, Shielding and Grounding

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Noise, Shielding and Grounding
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Characteristics of Electrical Noise

• Noise definition
• A stochastic interfering or modifying input (not
  the desired signal) in a system.
• Types
• Classification
• Source - man-made / natural
• Bandwidth narrow / broadband
• Coherency (has phase and frequency consistency)
• Reception mode radiated or conducted
• Inherent - generated within the elements of a
  circuit or system
• Examples
• thermal noise (Johnson noise)
• Current related junction noise (Shot noise)
• Interference - Generated external to the circuit
  or system
• Examples might be EMI (electromagnetic
  interference) from a radio station

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Characterization of Noise

• Signal to noise ratio
• Noise Figure for an amplifier circuit

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Noise terminology

• The noise voltage
• Equivalent short-circuit input RMS noise voltage
• The apparent noise voltage at the input of the
  noiseless amplifier with a shorted input
• nV per vHz or nV in a given frequency band
• . The noise current
• Equivalent short-circuit input RMS noise current
• The apparent noise current at the input of the
  noiseless amplifier due only to noise currents
• nA per vHz or nA in a given frequency band

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Inherent noise

• Generated within or by the device in question
• Types
• Thermal Noise - Johnson noise
• Function of thermally induced electron motion
• Gaussian amplitude distribution (white).
• Independent of direct-current flow.
• Calculated as Equation 4.88 in Fraden (Johnson,
  1928)
• Noise power (V2) is proportional to resistance
  (R), temperature (T), and bandwidth Df
• Estimated for resistances as
• Reduction of thermal noise for fixed R
• lower component temperature
• minimize bandwidth

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Inherent Noise - continued

• Types - continued
• Shot Noise - Current related noise in
  semi-conductors
• Proportional to junction current in a
  semiconductor
• Example The higher the bias current on a
  photodiode, the higher the shot noise will be.
• Calculation
• Caused by random arrival times of electrons in a
  current flow across a junction.
• Always associated with a direct-current flow.
• Proportional to the electronic charge and
  current.
• Gaussian amplitude distribution (white).
• Reduction of shot noise
• decrease currents
• design for narrow bandwidth

q electron charge Idc direct current Df
noise bandwidth
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Inherent Noise - continued

• Types - continued
• Pink Noise or 1/f Noise or flicker noise
• Noise that increases in magnitude with 1/f
• Associated with conduction
• Significant in semi-conductors, carbon film
  resistors, diodes, transistors, and light sources
• Associated with Flows of carriers in a
  discontinuous medium
• Contamination during manufacture increases this
  noise
• Dominates thermal noise below 100 hz
• Calculation
• Reduction of 1/f noise
• Reduce current
• Design for narrow bandwidth
• Use high quality components

K constant for a particular device I direct
current a constant in range 0.5 to 2 b
constant about unity Dfsmall bandwidth at
frequency f
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Minimization of inherent noise

• Operate circuits at low current levels
• Choose low-power technology CMOS, etc.
• Use low power op-amps
• Eliminate use of carbon film resistors in
  critical areas
• (carbon film has high 1/f noise)
• Operate circuits at low temperature
• Use cooling devices (note, thermal noise is
  proportional to absolute temperature)
• Purchase modern better quality components
• Design to use narrowest possible bandwidth

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EMI - ElectroMagnetic Interference

• EMI - Noise that is coupled into a system from
  external sources.
• Types (Coupling mechanisms)
• Galvanic coupling (Conductive coupling)
• Magnetic induction (inductive coupling)
• Electric induction - (capacitive coupling)

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Conducted EMC

• Conductive or galvanic connection
• Interference due to voltage drops in power and
  signal conductors.
• Example Through the power line or mains
• Voltage drops caused by current supplied to the
  EMI source load causes the supply voltage to the
  victim to vary. Load signals from the source
  are applied directly to the victims power supply.
• May occur in situations other than power
  distribution

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Reduction of conducted EMI

• Reduction of galvanic coupled interference
• minimize parallel connected systems
• Use capacitive de-coupling of the power supply
  for each component in parallel connected power
  distribution
• assure adequate capacity of conductors in
  parallel connected power distribution
• reduce power consumption

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Inductively coupled EMI

• Magnetic induction (inductive coupling)
• Coupling of source signals to victim system
  through a magnetic field. This effect occurs as
  a result of mutual inductance.
• where M mutual inductance between source and
  victim
• iL load side current of the inductive
  coupling
• M function of loop areas, loop orientations,
  magnetic screening

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Inductive coupling Analysis through Faradays Law
B Magnetic field T (or N/A m) A area of
coil m2 t time s V voltage v N number
of turns of coil

• Reduction of inductively coupled EMI
• Minimize area (rate of change of area)
• Minimize magnetic field (rate of change of
  magnitude and direction)
• Minimize number of turns (inductance)
• Common methods
• Use twisted pair wiring
• Use magnetic shielding
• Run conductor pairs close together on circuit
  boards

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Capacitively coupled EMI

• Electric induction - (capacitive coupling)
• Coupling of source signals to victim system
  through an electric field. This effect occurs as
  a result of a capacitive coupling.
• Where Cc is the capacitance of the coupling
  between source and victim and eL is the voltage
  drop between source and victim across the
  coupling.

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Reduction of capacitively coupled EMI

• Reduction of capacitively coupled EMI
• Reduce capacitance of coupling
• Cc depends on separation distance between
  plates, plate area, dielectric permittivity of
  the capacitors medium
• Increase plate separation
• Decrease plate area
• Decrease voltage level of source
• Common methods
• Separate victim circuit from EMI source
• Provide a conductive shield at low potential
  around victim and or source

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Grounding Issues

• Differences in ground level in systems can be
  an inherent source of interference.
• Voltage drop in the return across R1 causes
  signals from the source to appear to have a
  component of the power line current from the
  point of view of the victim
• Any duplicate low impedance ground path (ground
  loop) will have high current

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Ground loop example

• Consider a typical situation
• Leakage to the ground conductor in a system
  outside the measuring system
• Resistance between the load and the system ground
• Power connections in the system of interest at
  different locations

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Impact of ground differentials

• Measured voltage has a component due to leakage
• Reduction in the ground resistance (R2) in the
  system of interest reduces the voltage
  interference (DV)
• Current (i2) in the ground connection of the
  system of interest is increased significantly by
  decreasing (R2) and could be at destructive
  levels.